1. Field of the Invention
The present invention relates to an image forming apparatus such as a printer, a facsimile, and a copying machine.
2. Description of the Related Art
Conventionally known is an image forming apparatus including a plurality of image forming units for a plurality of colors including black. Each image forming unit forms a color image of its corresponding color on an image carrier therein (such as the one disclosed in Japanese Patent Application Laid-open No. 2006-201743).
The image forming apparatus disclosed in Japanese Patent Application Laid-open No. 2006-201743 has a direct transfer position where a black image formed in a black image forming unit is directly transferred onto a recording medium and a secondary transfer position where images of the other colors primarily transferred onto an intermediate transfer belt from the image forming units of the other colors are secondarily transferred onto the recording medium from the intermediate transfer belt. The secondary transfer position is located upstream of the direct transfer position in a recording medium conveying direction. The intermediate transfer belt is rotatably stretched across a plurality of roller members, and a driving roller that is one of the roller members causes the intermediate transfer belt to rotate. Furthermore, provided is a recording medium carriage belt that is rotatably stretched across a plurality of roller members and supports and conveys the recording medium to pass through the secondary transfer position and the direct transfer position. At the secondary transfer position, the driving roller that stretches the intermediate transfer belt, and a secondary transfer roller are arranged. The secondary transfer roller is one of multiple roller members stretching the recording medium carriage belt and faces the driving roller sandwiching therewith the intermediate transfer belt and the recording medium carriage belt. At the secondary transfer position, the outer circumferential surface of the intermediate transfer belt at the position where it is supported by the driving roller is brought into contact with the outer circumferential surface of the recording medium carriage belt at the position where it is stretched across the secondary transfer roller to thereby form a secondary transfer nip. With the recording medium carriage belt, the recording medium is passed through the secondary transfer position and the direct transfer position to superpose the images in other colors transferred on the recording medium at the secondary transfer nip formed at the secondary transfer position and the image in black color transferred on the recording medium at the direct transfer position on the recording medium to form a full color image on the recording medium. The recording medium with the full color image formed thereon is conveyed to a fixing unit provided at the downstream of the direct transfer position in the recording medium conveying direction, and the full color image on the recording medium is fixed onto the recording medium by the fixing unit.
However, in the image forming apparatus disclosed in Japanese Patent Application Laid-open No. 2006-201743, even when forming the image on the recording medium with only the image forming unit for black color, the recording medium supported by the recording medium carriage belt comes into contact with the intermediate transfer belt at the secondary transfer position. Therefore, the intermediate transfer belt is easily degraded, which shortens the life of the intermediate transfer belt.